1. Field of the Invention
The present invention relates to a power supply unit for generating a desired output voltage from an input voltage and electric equipment including the power supply unit and, more particularly, to a soft start circuit thereof.
2. Description of the Related Art
A conventional direct current stabilized power supply unit includes an error amplifier which amplifies a voltage difference between a feedback voltage which varies depending on the output voltage and a predetermined reference voltage and has an arrangement such that switching control of an output transistor is carried out using an output signal (error voltage) of the error amplifier. More specifically, such a direct current stabilized power supply unit has an arrangement such that a PWM (Pulse Width Modulation) signal of a duty depending on a result of the comparison between the error voltage and a predetermined slope voltage is generated and the output transistor is ON/OFF controlled using the PWM signal (see, for example, JPH7-336999 filed by the applicant of the present application).
Further, the conventional direct current stabilized power supply unit mentioned above includes a soft start circuit for preventing large current from flowing into the load when supplying power (excessively low output voltage). More specifically, the soft start circuit has such an arrangement that, as shown in FIG. 8, a soft start voltage Vss is generated that gradually rises after supplying power by introducing a predetermined constant current i1 (e.g., 100 nA) from a constant current source 11 to a capacitor Css, and PWM comparator PCMP generates a PWM signal of a duty depending on a result of the comparison between the lower of error voltage Verr or soft start voltage Vss and a predetermined slope voltage Vslp.
Certainly, with the conventional direct current stabilized power supply unit as mentioned above, a large current flowing into the load when supplying power can be prevented by using a soft start circuit.
However, with the conventional soft start circuit as mentioned above, it is difficult to cause the soft start voltage Vss to rise gradually due to the following reasons.
In order to attain a gradual rising of the soft start voltage Vss, the capacity of the capacitor Css should be increased or the constant current i1 flowing therein should be made small.
However, an increase in capacity of the capacitor Css needs a large occupied area and this is disadvantageous in promoting integration into LSI. For example, if one desires to reduce the rise of the soft start voltage Vss to 1/10 of the current state, the capacity (occupied area) of the capacitor Css should be expanded by approximately ten times, which suggests integration into LSI is no longer practical, and generally, there is no way to include such a large capacity capacitor Css unless it is mounted externally.
Furthermore, conventionally, a minute electric current i1 of approximately 100 nA is fed to the capacitor Css, and if this current should be reduced further, it poses a problem as how to maintain generation accuracy thereof. When output current of the constant current source I1 is merely reduced, maintaining generation accuracy of the constant current i1 is difficult, and in the worst case, there may be a possibility that a device is not started up at all.